1. Field of the Invention
The present invention relates to a thermosensitive recording material improved in image reliability (durability for discoloring or fading by plasticizer and oily materials) and in reliability in background area (for coloring or tinting of background area). Further the present invention relates to a thermosensitive recording material which is capable of exhibiting a significantly improved color-developing characteristics, with high reliabilities in said both image and background area. And more, the present invention relates to a synthesis method of an oligomer composition which is useful for the thermosensitive recording material.
2. Description of the Related Art
In general, the thermosensitive recording material has a thermosensitive recording layer which is mainly composed by an electron-donor colorless or lightly colored dye precursor as colorant and electron-acceptor compound as color-developer, and which is selectively heated by a thermal head, a thermal pen, laser beams or the like devices, thereby the dye precursor (leuco dye) and color-developer in heated point are instantaneously reacted with, causing recorded images, as described in Japanese Examined Patent Publications of Tokkou Shou 43-4160 and Tokkou Shou 45-14039 and other documents. These thermosensitive recording materials can record images thereon using a machine with relatively simple mechanism thus is easier in maintenance, lower in noises generated, therefore in recent years these thermosensitive recording materials being utilized as recording material for various and wide applications including computer, facsimile, automatic ticket issuing system, labeling system, thermal recorder and other machines.
As described above, the thermosensitive recording material uses in general a electron-donor colorless or lightly colored dye precursor as colorant and electron-acceptor compound as color-developer, therefore has a good appearance, an agreeable touch, and a high optical density of developed image, on the other hand it has drawbacks that the recorded images are apt to discolor or be lowered in optical density if they are contacted with the surfaces of plastic materials such as polyvinyl chloride sheet or film, owing to the action of plasticizer or additives contained therein, or they are easily declined in image density or easily colored or tinted in background area if the images are contacted with foods and cosmetics and the like, owing to the action of chemicals contained therein.
Accordingly, for improving the storage stability of recorded images of the thermosensitive recording material, many works to research and to develop favorable color developers have been extensively carried out, and many compounds of various types materials as developers have been proposed. One of such kind proposals have been made in recent year which is a proposal for employing a compound having urea group. For instances, Japanese Unexamined Patent Publications of Tokkai Hei 8-2110 and Tokkai Hei 8-2111 disclose the use of diphenyl-urea compounds. These compounds exhibit more excellent storage stability in the recorded images than that caused by conventional phenol developers, however they show a tendency declining the density of recorded image when the image is contacted with plasticizer, therefore it is difficult to say that these compounds result a sufficient image reliability. In addition, system by these compounds has a drawback that the developed color density is low, thus these compounds do not place the performance of image quality in real use level.
With regard to the use of a compound having urea group and urethane group, Japanese Unexamined Patent Publications of Tokkai Shou 8-2110 and Tokkai Shou 8-2111 disclose the use of a compound having urea-urethane group. This compound also causes more excellent storage stability of recorded images than that caused by conventional phenol developers, however it also shows a tendency declining image density when the image is contacted with plasticizer, therefore it is difficult to say that a sufficient reliability in image storage ability is attained by the use of the compound, thus similar as the case of aforementioned compounds, this compound does not make the arrival of image quality to real use level.
On the other hands, a compound having two urea groups and two salicylic acid residues in the molecule structure is disclosed in Japanese Unexamined Patent Publication of Tokkai Hei 8-244355, and a compound having two urea groups and two benzoic acid residues in the molecule structure is also disclosed in Japanese Unexamined Patent Publication of Tokkai Hei 8-197851, and a compound having two urea groups and two hydroxy phenyl residues in the molecule structure is also disclosed in Japanese Unexamined Patent Publication of Tokkai Hei 8-2109. These compounds show a tendency to improve image density owing to the influence of introduced groups which are other ones than the urea group and have a color-developing competency, however they also show a tendency declining image density when the image is contacted with plasticizer, therefore are it is difficult to say that sufficient image reliability can be caused by the use of these compounds, thus similarly as aforementioned compounds these compounds do not make the arrival of image quality to real use level.
Japanese Unexamined Patent Publication of Tokkai Hei 11-115314 discloses a use of poly-urea compounds derived from diisocyanate compounds and diamine compounds. These poly-urea compounds are composed in a polymer having large molecular weight, therefore they are low in color-developing competency and hence they are not ones having quality of practical use level.
Further Tokkai Hei 5-169836 discloses a compound having —SO2—NHCONH— group, and Tokkai Hei 11-263769 discloses a compound having —SO2—NHCONHCONH— group, both which are analogous to a compound having urea group. Improvements of image density were intended by these compounds, but the storage stability of developed image is not sufficient or coloring or tinting in back ground area is noticeable in either cases, hence they are not positioned in practical use level.
On the other hands, under the concept for applying the compound having an increased molecular weight, various kinds of high molecular (oligomer) of color-developers have been proposed. As typical examples, products of condensation reaction of 4-hydroxy benzoic acid with polyalcohols shown in Japanese Unexamined Patent Publication of Tokkai2001-30626 and WO99/51444 which corresponding to EPO 987122 can be instanced. However, these oligomer type color-developers cause insufficient results in both storage stabilities of images and in storage stability of background area, therefore are not suitable for real use. Still, there are provided cross-linked diphenyl sulfone compounds in a Japanese Unexamined Patent Publication of Tokkai Hei 10-29969. These compounds show a relatively high reliability in image stability but poor color-developing characteristics (image density and color-developing sensitivity), and they have a distribution of molecular weights and contain low molecular compounds therein which cause large fogging or tinting in background area, thus are not suitable for real use.
Furthermore, for the purpose to compensate drawbacks in above mentioned various developments, these are still proposed combination uses with one kind of compound as phenollic compounds, thereby color-developing characteristics (image density and color-developing sensitivity) and performance were intended to be improved, however in any case, as an inverse effect of improved color-developing characteristics, decreases in image reliability and in reliability of background area are induced, thus causing degradation from real use quality level.
Accordingly, hitherto, proposal of practical use level has not yet been attained in connection with color-developer showing excellent storage stabilities in both image and in background area, and showing excellent color-developing characteristics (image density and color-developing sensitivity).